Like the nervous system, the endocrine system regulates an animal's internal environment but it is a much slower control system. Chemical messengers (hormones) provide communication between sensory and effector cells. Hormone systems occur in all animals; they have become increasingly complex throughout evolutionary time compared to the basic neuron-endocrine systems of primitive animals. The endocrine system controls a wide range of physiological processes including reproduction, growth, development, metabolism, and osmo- and iono-regulation. It can respond to short- and long-term variations in internal and external environments and is important for the maintenance of homeostasis. Neuro-endocrine systems consist of neural sensory and interpretive pathways but instead of directly innervating an effector organ there is release of a chemical messenger into the blood at a hemal organ. This chemical messenger is then distributed to peripheral target organs where it has an effector action.
Hormones are secreted by endocrine glands (and neurohormones by nerve cells) in response to perturbing stimuli, and are then transported via the circulatory system or diffuse through tissues to target organs and cells. Thus a key characteristic of hormones is that they exert their action at a distance from the site of their secretion. Hormones do not initiate any unique cellular activities; rather they modify the rates of existing activities. Hormones may have inhibitory or excitatory effects on target cells, usually by inducing or repressing enzyme activity within cells, although they may act on the nucleus to influence the expression of genes or influence the permeability of cells to solutes.
Historically, hormones were considered to be chemicals released from endocrine glands (glands of internal secretion in contrast to exocrine glands such as salivary, sweat, and digestive glands that produce external secretions) but hormones may also be secreted by a variety of other tissues. Traditionally, hormones were considered to differ from neurotransmitters, which function only locally at the site of release (synapse) but this distinction is no longer so clear. Hormones function at very low concentration (e.g., 10~12-10~9 M). Target organs have specificity for particular hormones due to the properties of receptors that are either on the surface of the cell membrane or inside the cell. Receptors reversibly bind the hormones with high specificity and affinity. Water-soluble hormones are derivatives of amino acids (catecho-lamines, peptides, proteins) or fatty acids (eicosinoids). These interact with surface receptors that span the cell membrane. Often they trigger a secondary 'messenger' inside the cell. In contrast, lipid-soluble hormones such as steroids (adrenocortical and gonadal steroids in vertebrates, ecdysones, and juvenile hormones in invertebrates)
and thyroid hormones usually pass through the cell membrane and interact with intracellular receptors. Some bind to membrane receptors which are then internalized. Many hormones that are transported in the circulatory system (in particular the lipid-soluble hormones) bind to a water-soluble carrier protein to aid transport.
Hormones are classified by the distance over which they travel to have their effect (Figure 3). Autocrine hormones affect the cell that secreted them. They react with receptors on their own surface to produce a response and are usually involved in cell division. Paracrine hormones act over a very short distance, diffusing through extracellular fluid to affect local tissues. Endocrine hormones affect distant organs and tissues. They are secreted into the circulatory system and are transported by the hemolymph or blood. Pheromones are an additional form of chemical communication that occurs between rather than within individuals. They are highly volatile compounds released into the external environment and detected in small concentrations by receptors (usually on the nasal epithelium of vertebrates or antenna of insects) of another individual. Pheromones
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